1. Field of the Invention
The present invention relates to a method and an appropriate apparatus for charging rechargeable batteries.
Various rechargeable batteries, such as lead-acid or lithium-ion rechargeable batteries, need to be charged using a voltage source whose current is limited, in which the voltage is regulated very accurately and, in some circumstances, must also be varied as a function of the temperature of the rechargeable battery cells, in order to compensate for temperature-dependent fluctuations during the charging process. In this case, the voltage source makes use of current limiting, which limits the charging current supplied from the voltage source. The current limiting is in this case particularly effective when a completely discharged rechargeable battery is connected to the voltage source. FIG. 2 illustrates a conventional method for charging rechargeable batteries, and the way in which the current limiting operates. As shown in FIG. 2, a charging voltage UL which is less than the maximum permissible charging voltage Umax is initially applied to the rechargeable battery. As the charging process progresses, the charging voltage is raised and increases up to the regulator voltage Umax setting. The voltage from the voltage source UL is then limited to this preset value Umax by means of the voltage regulation setting. The internal rechargeable battery voltage, that is to say the no-load voltage of the rechargeable battery, which is produced as a result of the charging process, rises as the charging process progresses. Conversely, this means that, since the charging voltage UL is now kept constant and, to a first approximation, the internal resistance of the rechargeable battery is assumed to be constant, the current drawn or the charging current IL of the rechargeable battery falls continuously from the time T2 at which the voltage limiting starts. The charge state of the rechargeable battery can thus be deduced from the magnitude of the charging current IL. Conversely, the charging current IL must be monitored in order to detect when it falls below a predetermined lower limit value, in order in this case to terminate the charging process, and to switch off the charging voltage source, immediately or after a specific time.
The charging voltage source may be in the form of a high-precision voltage source (in the form of a linear or switched-mode regulator) with current limiting. This high-precision voltage source is accommodated in the immediate vicinity of the rechargeable battery, that is to say very close to the rechargeable battery being charged, or can be connected to the rechargeable battery via so-called sense lines. These sense lines are lines that are provided in addition to the current-carrying lines which are used for charging the rechargeable battery. The sense lines do not carry current, and are intended exclusively for monitoring the charge state of the rechargeable battery. However, this procedure has the disadvantage that, despite its space requirement and its heat losses, the charging voltage source must be accommodated close to the rechargeable battery to be charged, or the wiring complexity and/or the complexity of the plug connections are/is increased by the use of the sense lines.
The charging current is measured, in a known manner, by inserting a shunt resistor into the charging circuit and by measuring the voltage dropped across this resistor. The use of such a shunt resistor in turn results in the disadvantage that it causes an additional voltage drop in the charging circuit, which slows down the charging process. Furthermore, the monitoring of the voltage dropped across the shunt resistor represents a level of complexity which is not insignificant for measured-value detection. This is particularly true if the shunt resistor needs to be inserted in the positive line rather than in the ground line (for example if the rechargeable battery is coupled to ground via the appliance to be operated).
Furthermore, the conventional procedure described above results in the problem that different types of rechargeable batteries (for example NiCd/NiMH or Li-Ion rechargeable batteries) cannot be charged at the same time since their final charging voltages are of different magnitude. Generally, NiCd/NiMH rechargeable batteries are charged with a constant current, independently of the charging voltage.
A further known option for charging rechargeable batteries is to use a regulated, that is to say variable, current source, in which case the charging current supplied to the rechargeable battery is regulated by varying the duty ratio of a pulsed signal which is applied to a switch provided in a current-carrying conductor. In particular, the internal rechargeable battery voltage of the rechargeable battery is monitored, and the duty ratio, that is to say the pulse-pause ratio of the switch, and hence the charging current as well, are regulated as a function of this. A battery charger of this type is described in U.S. Pat. No. 4,885,522. In the circuit described there, the emitter-collector path through a transistor which is used as a line circuit-breaker, and an inductance in series with it, are connected in the charging circuit. The base of this switching transistor is connected to the output of a Schmitt trigger. In operation, this circuit represents an oscillator, which uses the switching transistor to produce a pulsed charging current. The duty ratio of the pulsed charging current is governed by the hysteresis of the Schmitt trigger, which is in turn governed by the switching state of a temperature filter connected to the battery to be charged. This circuit allows high switching frequencies to be achieved, and thus makes it possible to reduce the amount of heat produced in the switching transistor. This concept admittedly allows different rechargeable battery technologies (for example NiMH or Li-Ion rechargeable batteries) to be charged without any additional complexity arising from variable current limits in one and the same appliance. However, current overshoots occur at the switching time of the switch and can lead to a short-term overvoltage being applied to the rechargeable battery at the switching times, and because of the regular pulsed operation, this would lead to the rechargeable battery being damaged over a period of time. Increased efforts are therefore required for the design of the electronics for the current source used, in order to limit the previously described current overshoots at the switching times, although this is associated with increased costs.
It is accordingly an object of the invention to provide a method and an appropriate apparatus which overcome the above-mentioned disadvantageous of the prior art methods and apparatus of this general type, and which enable rechargeable batteries to be charged reliably with little complexity.
In particular it is intended to be possible to charge rechargeable batteries based on different rechargeable battery.
With the foregoing and other objects in view there is provided, in accordance with the invention a method for charging a rechargeable battery, that includes steps of: charging a rechargeable battery with an unpulsed rated current supplied from a variable current source; detecting at least one operating parameter of the rechargeable battery; providing a pulsed control signal having a specific duty ratio and producing the duty ratio as a function of the operating parameter; and adjusting the unpulsed rated current supplied from the current source as a function of the duty ratio of the pulsed control signal.
In accordance with an added mode of the invention, a rechargeable battery voltage that is produced as a consequence of charging the rechargeable battery is detected; and the rechargeable battery voltage is used as the operating parameter.
In accordance with an additional mode of the invention, the duty ratio of the pulsed control signal is increased if the detected rechargeable battery voltage of the rechargeable battery is less than a desired charging voltage; and the duty ratio is decreased if the detected rechargeable battery voltage of the rechargeable battery is greater than the desired charging voltage.
In accordance with another mode of the invention, the rechargeable battery voltage is digitally averaged before evaluating the rechargeable battery voltage.
In accordance with a further mode of the invention, the duty ratio of the pulsed control signal is adjusted as a function of a current characteristic that is specific to the current source.
In accordance with a further added mode of the invention, the method includes steps of: before starting the step of charging the rechargeable battery, measuring the input impedance of the current source to determine a type of the current source; and basing the charging on a current characteristic that corresponds to the determined type of the current source.
In accordance with a further additional mode of the invention, the duty ratio of the pulsed control signal is adjusted as a function of a type of rechargeable battery.
In accordance with a further still added mode of the invention, the temperature of the rechargeable battery is detected; and the detected temperature is used as the operating parameter.
In accordance with a further still additional mode of the invention, the step of determining the operating parameter of the rechargeable battery is performed at regular intervals.
In accordance with a further still mode of the invention, the unpulsed rated current of the current source is increased as the duty ratio of the pulsed control signal increases.
In accordance with yet an added mode of the invention, a difference between the detected rechargeable battery voltage and the desired charging voltage is obtained; and the step of varying the duty ratio is performed by changing the duty ratio in coarser steps as the difference increases.
In accordance with yet an additional mode of the invention, a rechargeable battery voltage that is produced as a consequence of charging the rechargeable battery is detected; the rechargeable battery voltage is used as the operating parameter; the duty ratio of the pulsed control signal is varied by: increasing the duty ratio of the pulsed control signal if the detected rechargeable battery voltage of the rechargeable battery is less than a desired charging voltage, and reducing the duty ratio if the detected rechargeable battery voltage of the rechargeable battery is greater than the desired charging voltage; a difference between the detected rechargeable battery voltage and the desired charging voltage is obtained; and the step of varying the duty ratio by changing the duty ratio is performed in coarser steps as the difference increases.
In accordance with yet an added mode of the invention, upon initiating the charging, the duty ratio of the pulsed control signal is set to a minimum value.
In accordance with yet another mode of the invention, each duty ratio value of the pulsed control signal is uniquely associated with a specific value of the rated current supplied by the current source.
In accordance with yet another added mode of the invention, the duty ratio of the pulsed control signal is adjusted as a function of the detected operating parameter such that the rated current supplied by the current source is varied in accordance with a square law.
With the foregoing and other objects in view there is provided, in accordance with the invention an apparatus for charging a rechargeable battery, that includes: a variable current source for providing an unpulsed rated current for recharging a rechargeable battery; a detection device for detecting at least one operating parameter of the rechargeable battery; a control device for producing a pulsed control signal having a specific duty ratio formed as a function of the detected at least one operating parameter of the rechargeable battery; and a current source control device for detecting the duty ratio of the pulsed control signal from the control device and for producing an adjustment signal for the variable current source such that the unpulsed rated current supplied by the current source is adjusted as a function of the duty ratio of the pulsed control signal.
In accordance with an added feature of the invention, the detection device is constructed to detect a rechargeable battery voltage of the rechargeable battery that is produced as a consequence of a charging process; and the rechargeable battery voltage is one of the at least one operating parameter of the rechargeable battery.
In accordance with an additional feature of the invention, an analog/digital converter is connected between the detection device and the control device.
In accordance with another feature of the invention, the variable current source has a specific current characteristic; and the control device is designed to adjust the duty ratio of the pulsed control signal to match the current characteristic that is specific to the variable current source.
In accordance with a further feature of the invention, the current source has an input impedance and a type; the control device includes a current characteristic memory for storing current characteristics for various current source types; the control device is designed to measure the input impedance of the current source before charging the rechargeable battery; and the control device is designed to deduce the type of the current source as a function of the measured input impedance so that one of the current characteristics that corresponds to the deduced type of the current source can be used as a basis for subsequently charging the rechargeable battery.
In accordance with a further added feature of the invention, a control line supplies the pulsed control signal from the control device to the current source control device; and the control device is constructed to measure the input impedance of the current source using the control line.
In accordance with a further additional feature of the invention, the detection device includes a temperature detection device for detecting a temperature of the rechargeable battery as one of the at least one operating parameter; and the control device is constructed to adjust the duty ratio of the pulsed control signal by taking account of the detected temperature of the rechargeable battery.
In accordance with yet an added feature of the invention, the rechargeable battery has a type; the detection device includes a rechargeable battery type identification means for identifying the type of the rechargeable battery; and the control device is constructed to adjust the duty ratio of the pulsed control signal by taking account of the identified type of the rechargeable battery.
In accordance with yet an additional feature of the invention, the pulsed control signal has a plurality of duty ratio values and each one of the plurality of the duty ratios has a uniquely associated corresponding value of the rated current of the current source.
In accordance with yet another feature of the invention, the apparatus is used in combination with an appliance that is operated with the rechargeable battery, and the control device is constructed in the appliance.
In accordance with yet another added feature of the invention, the appliance is a mobile telephone.
In accordance with yet another additional feature of the invention, the control device includes a microcontroller.
According to the invention, a pulsed control signal with a specific duty ratio is admittedly produced as a function of an operating parameter, for example the instantaneous charge state of the rechargeable battery to be charged, in an analogous manner to the second known solution described above. However, in contrast to the solution already described, this pulsed control signal is not used for pulsing the charging current but for setting the rated current supplied from a controllable, that is to say variable, current source as a function of the duty ratio. Thus, in the solution according to the invention, the rechargeable battery is not supplied with a pulsed charging current and there are no switching processes in the current-carrying lines, so that the present invention overcomes the problems associated with the second known solution described above.
Furthermore, in the present invention, the current of the current source is not adjusted by analog voltage limiting of the charging current source, but by the charging process being controlled with the aid of the duty ratio of the pulsed control signal. In contrast to the already described, known xe2x80x9cpulsedxe2x80x9d solution, the current limit of the controllable current source in the present invention corresponds to the mean charging current.
The pulsed control signal can, possibly after conversion to a reference voltage, be passed via a low-pass filter or the like, resulting in an analog voltage which corresponds to the desired charging current. This analog voltage is then used as a nominal variable for open or closed loop control of the current from the current source, in order to adjust the rated current supply from the current source, as appropriate. In this case, each duty ratio of the pulsed control signal is uniquely associated with a corresponding instantaneous value of the rated current. That is to say the magnitude of the current that is forced to flow from the current source is defined uniquely by the duty ratio of the pulsed control signal and is thus known to the charge controller, without having to be measured directly. Firstly, this reduces the circuit complexity and, secondly, it avoids any voltage drop across a shunt resistor which would otherwise have to be provided in order to measure the current in the charging circuit (see the first known solution described above). Varying the current limit of the current source avoids the possibility of an excessively high voltage occurring between the terminals of the rechargeable battery towards the end of the charging process, due to the internal resistance of the rechargeable battery.
The present invention has the advantage that the overall complexity of the current source, and the costs associated with it, are reduced in comparison to those for the known solutions described above.
Further advantages of the present invention are that there is no need for a high-precision voltage/current regulator, and no power losses occur in the appliance. Different rechargeable batteries can be charged without any additional hardware complexity. Any converters or measurement capabilities which may be present for determining the voltage of the rechargeable battery may still be used. No additional hardware complexity is required when using different final charging voltages for Li-Ion rechargeable batteries.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and apparatus for charging rechargeable batteries, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.